At present, the methods used for purifying industrial process water call for an initial step in which the waste is homogenised and in which coarse purification is performed by mechanical separation (sieving, decanting, etc.). This is followed by a second step in which pollution is reduced by simple methods which may be biological or physico-chemical (eg. coagulation, flocculation and filtration), and there may be an optional third or finishing step in which residual dissolved matter is eliminated by means of fine separation techniques such as: separation by means of semi-permeable membranes, adsorption on the surface of a solid of high specific surface area, ion exchange on a resin, or separation by means of grafted cellulose.
For waters having a high chemical demand for oxygen (CDO) and having fine particles in suspension, eg. the waters from a dyeing works where the particles in suspension are about 1 to 2 tenths of a millimeter in diameter, coagulation-flocculation processing is very important. But regardless of the conditions under which this chemico-physical process takes place, in particular regardless of the degree of separation attained by a prior decanting step, the CDO after coagulation-flocculation processing is not reduced by more than 40 to 50%.
The chemical demand for oxygen of water treated in this way is generally much too high for the water to be recycled in the factory's industrial process from whose treated wastes the water originates. This leads to high water supply costs together with relatively high tax payments for rejecting treated water into protected water courses.
Further, and particularly for water from dyeing works, conventional processing does not remove the color from the waste water, which makes the waste water quite unusable in industrial processes.
Another problem inherent to usual purification techniques lies in getting rid of the resulting sludge at the end of the process. Waters having a high chemical demand for oxygen and which therefore require large quantities of coagulating agent and of flocculating agent give rise to considerable volumes of sludge.
Preferred embodiments of the invention provide a method of purifying waste water which method reduces the pollution in the water sufficiently for the water to be recycled in the industrial process. More particularly, the invention can be used to remove the color from waste water from dyeing works.
Furthermore, the invention can be used to obtain the polluting matter from the water in a highly concentrated form.
Finally the invention can be used to provide a method and an installation for purifying waste water that are simple and cheap.